Conventional multiplexing methods can be mainly classified into Orthogonal Time Division Multiplexing (OTDM), Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Code Division Multiplexing (OCDM).
OTDM is a method in which each of frames 108 and 109, which is composed of a plurality of time slots T0(100)˜T7(107), as illustrated in FIG. 1a. Channels are distinguished by the time slots of a frame, and one time slot per frame is assigned to each of the second communication stations MS0˜MS7, and thereby a plurality of second communication stations can simultaneously communicate. Therefore, the number of channels is determined by the number of time slots within a frame, and the number of second communication stations which can simultaneously communicate is also determined by the number of time slots within a frame.
OFDM is a method in which an available frequency bandwidth is divided into a plurality of frequency bands F0(110)˜F7(117), as illustrated in FIG. 1b, channels are distinguished by the frequency bands, and one of the divided frequency bands is assigned to each of the second communication stations MS0˜MS7, and thereby a plurality of second communication stations can simultaneously communicate. Therefore, the number of channels is determined by the number of frequency bands, and the number of second communication stations which can simultaneously communicate is also determined by the number of frequency bands.
OCDM is a method in which spread spectrum is used, channels are distinguished by orthogonal codes C0(120)˜C7(127), as illustrated in FIG. 1c, and one orthogonal code is assigned to each of second communication stations MS0˜MS7, and thereby a plurality of second communication station can simultaneously communicate. Therefore, the number of channels is determined by the number of orthogonal codes, and the number of second communication stations which can simultaneously communicate is also determined by the number of orthogonal codes.
FIGS. 2a to 2c are schematic block diagrams of multi-user communication systems using conventional multiplexing methods. FIG. 2a is a block diagram of a multi-user communication system using Orthogonal Time Division Multiplexing (OTDM), in which messages Me0˜Me7 to be transmitted to second communication stations are converted into the symbols of a signal constellation through encoders 201, interleavers 202, and symbol mappers 203 for mapping symbols according to the signal constellation to be transmitted by a physical layer. The signal constellation may be associated with QAM, QPSK, BPSK or some other signal constellation. The converted symbols Se0˜Se7 are added to each other through burst formatters 210 for enabling the symbols to be inserted into the time slots assigned to respective second communication stations. At this time, the burst formatters 210 recognize the locations of time slots to be used by themselves through control signals. The signals are added to each other, pass through an analog control part 213 including a DAC 211 for converting a digital signal into an analog signal, and a Radio Frequency (RF) unit 212 for converting the baseband signal to a carrier frequency band, and are transmitted to the second communication stations through an antenna. Each second communication station must extract only a signal transmitted to itself from the multiplexed and received signals. The received signal is converted from the carrier frequency band into a baseband in a RF unit 214 and is converted from analog signal to digital signal in Analog/Digital Converter(ADC) 215. The second communication station extracts only a signal Se0 in a designated time slot from the signal passing through an Analog Control Part 216 consisting of an RF unit 214 and an ADC 215 using a burst synchronization unit 217. In the example of FIG. 1a, the second communication station receiving a signal Me0 is only shown, but the remaining second communication stations also perform the same function. The extracted signal passes through a symbol demapper 204, a deinterleaver 205, and a decoder 206, so that the transmitted message Me0 is recovered.
FIG. 2b is a block diagram of a multi-user communication system using Orthogonal Frequency Division Multiplexing (OFDM), in which messages Me0˜Me7 to be transmitted to second communication stations are converted into the symbols Se0˜Se7 of a signal constellation through encoders 201, interleavers 202, and symbol mappers 203, like OTDM. The converted symbols Se0˜Se7 pass through a frequency selector 221 for coupling the symbols to taps assigned to the respective frequency bands of an Inverse Discrete Fourier Transform (IDFT) or Inverse Fast Fourier transform (IFFT) unit 222. The parallel signals from the IDFT or IFFT unit are converted into a serial signal (parallel-to-serial converter 233). Then, in order to decrease interference, a guard interval is added to the serial signal. Thereafter, the serial signal passes through an analog control part 213, and is transmitted via an antenna. Each of the second communication stations eliminates the guard interval from a received signal (225), converts the serial signal to parallel signals (226), performs DFT or FFT (227), and extracts only the symbol Se0 assigned to a designated frequency band according to a signal F0, which is assigned from the first communication station, and the received symbol passes through a symbol demapper 204, a deinterleaver 205, and a decoder 206, and then, the transmitted message Me0 is finally recovered.
FIG. 2c is a block diagram of a multi-user communication system using Orthogonal Code Division Multiplexing (OCDM), in which messages Me0˜Me7 to be transmitted to second communication stations are converted into the symbols Se0˜Se7 of a signal constellation through encoders 201, interleavers 202, and symbol mappers 203, like other multiplexing methods. The converted symbols Se0˜Se7 are spread by orthogonal codes assigned to respective second communication stations (spreader 220), and pass through an analog control part 213 and are transmitted through an antenna. Each of the second communication stations extracts symbols transmitted to itself from signals received through an analog control part 216 by using a rake receiver 221. At this time, the second communication station uses an orthogonal code assigned to itself. The detail operation of the rake receiver is already known. The symbols received through the rake receiver are recovered to the originally transmitted message Me0 through a symbol demapper 204, a deinterleaver 205 and a decoder 206.
The above-described conventional multiplexing methods assign orthognal resources (time slots in OTDM, frequency bands in OFDM, and orthogonal codes in OCDM) to second communication stations, and thereby they enable a plurality of second communication stations to simultaneously communicate. However, the number of orthogonal resources is limited to N, so that the number of second communication stations which can simultaneously communicate is also limited to N (in the example of FIG. 2, N is 8). Furthermore, in a system using a conventional multiplexing method, once a call is connected, a second communication station continues to occupy assigned resources until the call is terminated, so that a new call can not be connected if an (N+1)th second communication station requests the connection of a new call. However, messages are not always exchanged during a call connection. Particularly, in the case of Internet data, the probability that there exist messages to be transmitted at a certain time is low because of bursty characteristics of data traffic. That is, although a second communication station occupies orthogonal resources, as shown in FIGS. 3a, 3b and 3c, a large amount of unused time slots or frequency bands may occur. That is, a large amount of orthogonal resources can be wasted. In order to reduce the waste of orthogonal resources, conventional multiplexing methods must use fast assignments and releases of resources, but a certain amount of the limited resources is assigned to control information because of the transmission and reception of control signal information for frequent resource assignments and releases.